An exhaust gas discharged from diesel engines contains particulate matter (PM) comprising as main components carbonaceous soot and soluble organic fractions (SOFs) comprising high-boiling-point hydrocarbon components, which are likely to adversely affect humans and environment when discharged into the air. Accordingly, ceramic honeycomb filters have conventionally been attached to exhaust pipes of diesel engines for removing PM.
One example of ceramic honeycomb filters for capturing PM in the exhaust gas is shown in FIGS. 1 and 2. The ceramic honeycomb filter 10 comprises a ceramic honeycomb structure comprising porous cell walls 2 defining a large number of outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4 and a peripheral wall 1, and upstream-side plugs 6a and downstream-side plugs 6c sealing the exhaust-gas-inlet-side end surface 8 and exhaust-gas-outlet-side end surface 9 of the flow paths 3 and 4 alternately in a checkerboard pattern. The peripheral wall 1 of the ceramic honeycomb filter is fixed by grip members (not shown) of metal meshes or ceramics mats, etc. to prevent movement during operation, and disposed in a metal container (not shown).
In the ceramic honeycomb filter 10, an exhaust gas is cleaned as follows. As shown by dotted arrows in FIG. 2, an exhaust gas flows into the outlet-side-sealed flow paths 3 opening on the inlet-side end surface 8. While passing through the cell walls 2, particularly through communicating pores on and in the cell walls 2, PM in the exhaust gas is captured. The cleaned exhaust gas flows from the inlet-side-sealed flow paths 4 opening on the exhaust-gas-outlet-side end surface 9 to the air.
PM continuously captured by the cell walls 2 clogs communicating pores on and in the cell walls 2, resulting in increased pressure loss when the exhaust gas passes through the ceramic honeycomb filter. Accordingly, PM should be burned off to regenerate the ceramic honeycomb filter before the pressure loss reaches a predetermined level.
The ceramic honeycomb filter should meet the requirements of a high capturing ratio of fine particles and low pressure loss. However, because these requirements are in a contradictory relation, the optimization of the porosity, volume, size, etc. of pores on the cell wall surface to meet both requirements have conventionally been investigated.
JP 2005-530616 A discloses a ceramic filter constituted by a cordierite honeycomb structure with ends plugged for capturing and burning fine particles discharged from diesel engines, d50/(d50+d90) determined from a pore diameter distribution being less than 0.70, a permeability factor Sf when soot is accumulated, which is defined by the formula of [d50/(d50+d90)]/[porosity (%)/100], being less than 1.55, and a thermal expansion coefficient (25° C. to 800° C.) being 17×10−7/° C. or less, describing that with such a pore structure (pore size distribution and the communications of pores), small pressure loss can be kept even when carbon soot is accumulated.
JP 2002-219319 A discloses a porous honeycomb filter formed by a material whose main crystal phase is cordierite having a controlled pore size distribution, the pore size distribution being such that the volume of pores having diameters of less than 10 μm is 15% or less of the total pore volume, the volume of pores having diameters of 10-50 μm is 75% or more of the total pore volume, and the volume of pores having diameters exceeding 50 μm is 10% or less of the total pore volume. JP 2002-219319 A describes that because of the above pore size distribution, this porous honeycomb filter has high efficiency of capturing PM, etc., with suppressed pressure loss increase due to the clogging of pores. JP 2002-219319 A also describes that such pore size distribution can be controlled by adjusting the particle size of a silica component, one of cordierite-forming materials, and by lowering the concentration of kaolin.
JP 61-129015 A discloses an exhaust-gas-cleaning filter having small pores having diameters of 5-40 μm and large pores having diameters of 40-100 μm on at least inlet path surfaces of cell walls, the number of small pores being 5-40 times that of large pores, pores on the surface communicating with pores inside the cell walls. JP 61-129015 A describes that this exhaust-gas-cleaning filter always exhibits high, substantially constant efficiency of capturing fine particles.
JP 2003-40687 A discloses a ceramic honeycomb structure composed of cordierite as a main component, and having porosity of 55-65% and an average pore diameter of 15-30 μm, the total area of pores opening on the cell wall surface being 35% or more of the total cell wall surface area. JP 2003-40687 A describes that this honeycomb ceramic structure exhibits high capturing efficiency with low pressure loss.
JP 2002-355511 A discloses an exhaust-gas-cleaning filter comprising a ceramic honeycomb structure having a catalyst carried on the cell wall surface, the cell walls having porosity of 55-80%, and the total area of pores opening on the cell wall surface being 20% or more of the total cell wall surface area. JP 2002-355511 A describes that with increased contact area between the catalyst carried on the cell walls and the accumulated PM, this exhaust-gas-cleaning filter exhibits high performance of oxidizing PM by the catalyst with suppressed pressure loss increase.
JP 2002-349234 A discloses an exhaust-gas-cleaning filter having a catalyst carried, the total area of pores opening on the cell wall surface being 30% or more of the total cell wall surface area, the total opening area of large pores having opening diameters of 30 μm or more being 50% or more of the total opening pore area. JP 2002-349234 A describes that such structure provides drastically improved burning efficiency of PM, while preventing damage due to thermal stress.
However, the exhaust-gas-cleaning filters described in JP 2005-530616 A, JP 2002-219319 A, JP 61-129015 A, JP 2003-40687 A, JP 2002-355511 A, and JP 2002-349234 A exhibit PM-capturing performance, which becomes high by the accumulation of PM to some extent, but is not necessarily sufficient at an early stage of use before PM is accumulated (when the ceramic honeycomb filter starts to be freshly used or reused after regeneration). Particularly they fail to capture harmful, nano-sized PM sufficiently, but discharge it, causing a serious problem under the strengthened exhaust gas regulations.
To solve these problems, JP 2004-360654 A discloses a ceramic honeycomb filter whose cell walls have porosity of 55-75% and an average pore diameter of 15-40 μm, the total area of pores opening on the cell wall surface being 10-30% of the total cell wall surface area, and pores having equivalent circle diameters of 5-20 μm being 300/mm2 or more among those opening on the cell wall surface. However, even the ceramic honeycomb filter described in JP 2004-360654 A fails to solve the problem of low PM-capturing efficiency at an early stage of use after its regeneration.
As a method for producing a porous ceramic structure having stable porosity, JP 2007-45686 A discloses the use of porous resin particles having an average particle size of 10-50 μm and porosity of 50-90% as a pore-forming material. JP 2007-45686 A describes that the use of porous resin particles generating less heat when burned than solid particles and more resistant to collapse than hollow particles as a pore-forming material can suppress the collapse of pore-forming material particles during the mixing and blending of the molding material, and excessive heat generation during sintering, thereby producing a porous ceramic structure with stable porosity at a high yield. However, when the porous resin particles are used as a pore-forming material, high pressure is needed in the extrusion molding because of friction resistance among the pore-forming material particles, likely resulting in the deformation of extrudates and dies.